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Type:
Bug
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Resolution: Fixed
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Priority:
Major - P3
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Affects Version/s: None
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Component/s: None
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Labels:None
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Fully Compatible
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ALL
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QE 2023-03-06
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35
Currently, $count aggregation stage will produce a project stage for the group id, even when the group id is constant.
For aggregation:
db.coll.explain().aggregate([{$count: "n"}]).stages[0].$cursor.queryPlanner.winningPlan.slotBasedPlan.stages
It produces:
[2] mkobj s9 [_id = s6, n = s7] true false [2] group [s6] [s7 = sum(1)] spillSlots[s8] mergingExprs[sum(s8)] [2] project [s6 = (null ?: null)] [1] scan s4 s5 none none none none [] @"e8cac054-9809-447e-94d0-1225895e2c70" true false
The project stage is evaluated for every document and is not necessary.